Updating a PC or laptop's graphics card can be a cost-effective upgrade. A graphics card upgrade allows your PC to play games smoothly at higher resolutions, with more special effects, but there are plenty of options and potential pitfalls to consider.

A powerful graphics card is an essential component in any high-performance desktop or even laptop PC. A standalone card is required in order to handle the complex 3D maths and geometry that go on behind the scenes to add realism to games and HD content. ATI (owned by CPU maker AMD) and nVidia release a new generation of their graphics cards every year or so, with refined hardware and more transistors squeezed in, and here's our guide to selecting the best for your requirements.

Substituting one brand of card for another with the same graphics chip and memory allocation almost certainly won't affect performance, but individual card manufacturers try to squeeze out an advantage by producing graphics cards with subtly different specifications, so it is important to know what to look for.

The key feature of any graphics card is the graphics processor, more commonly referred to as the GPU. There are two names you need to know in graphics processors: ATI and nVidia. Much like the Intel and AMD duopoly in PC processors, these two manufacturers produce the chips around which others build graphics cards. See also: Group test - what's the best graphics card?

Put simply, the GPU determines the quality of images displayed, as well as their refresh speed. At the time of writing the nVidia GTX 570 is the highest-performing card we're regularly seeing in our tests, while the 460 and Radeon HD 6850 represent excellent value.

Generally speaking, the more powerful the GPU the better - gamers may have to switch off some detail settings with a cheaper card, for instance. See also: Group test - what's the best budget graphics card?

The next most important factor is dedicated video memory, or RAM. Even those on a budget should be able to get a minimum of 1GB of DDR memory in a standalone graphics card. For high-end cards, look for 1,280MB or even more - we're seeing cards with 2GB coming through our labs.

Most decent graphics chips now come with GDDR5 RAM. This is more sophisticated than GDDR3 and will in effect quadruple the clock speed, whereas GDDR3 merely doubles it.

Another important factor is the memory interface (or bus), which is determined by the chip. This governs how much data can be sent through at once. A 256bit interface can let through twice as much data as a 128bit interface. The memory can work at higher speeds, as determined by the clock speed. The cards with the best combination of memory interface size and clock speed should produce the best performance.

A decent measure of this is the memory bandwidth, which is the memory interface divided by eight, multiplied by the effective memory clock speed. If the memory is GDDR5, you get the effective speed by quadrupling the basic memory clock; for GDDR3 you double it. Measured in gigabytes per second (GBps), the higher the memory bandwidth, the better the card will be able to handle textures and other features.

Stream processors are a significant feature of today’s cards, able to work as either vertex or pixel shaders. Older graphics chips used dedicated vertex shaders and pixel shaders, but if a game used more of one than the other, the card wasn’t used to its full potential. When an object has been ‘created’ by the shaders, a texture or image will be applied to it. Texture units work with the shaders to apply textures as quickly as possible.

This is a resource-intensive process, so the more texture units, the better the fill rate will be. We calculate the fill rate by multiplying the number of texture units with the core clock speed of the card. The higher the resulting number of texels (texture elements), the more textures the card can fetch. Raster output units (ROPs) are also important since these turn the information from vertex and pixel shaders to complete pixels to create the final image.

DirectX 11.0 support is required for advanced, modern games; cheaper cards are likely to lack the firepower to do justice to DirectX 11.0 games, however.

Finally, one thing that is often overlooked but could be crucial, is the practical consideration of fitting a graphics card. You need to make sure it will physically fit - not all cards fit in all cases - and that you have sufficient connections from the PSU (power supply unit).

The cheapest cards require no additional power, but most cards require one six-pin connector and some will need two. The PSU should be rated high enough to handle the demands of the graphics card.

Every card now has a digital video port. Most have DVI and/or Display Ports, and the best cards also offer HDMI. If you plan to use ATI's EyeFinity, which stretches your display over multiple monitors, you’ll need to ensure your card has enough ports and connectors.

For more information on PC upgrades, visit Upgrade Advisor.